In an ordinary dry kiln system, relatively dry outside air is vented into the kiln and then heated. Often, a boiler is used to heat the air. The heated air is then passed over an assemblage of space-stacked lumber. Moisture from the lumber evaporates into the heated air, thereby lowering the moisture content of the lumber. The heated air then contains the released moisture, typically in the form of steam. Finally, the moisture-laden air is directed out of the kiln. This configuration is sometimes called an open system, because fresh dry outside air is continuously applied to the system, while moisture-laden air generated within the kiln is exhausted from the system.
Open systems, however, require significant amounts of energy to heat the fresh outside air, since energy is necessarily lost along with the exhausted moisture-laden air. Those of skill in the art will appreciate that a more closed system approach might be possible if moisture can be economically removed from the air and returned to the kiln. Further, recovered moisture, being relatively pure, may be used for other purposes, such as replenishing a supply of boiler water.